1. Field of the Invention
This invention relates to an image forming apparatus such as a copying apparatus, a laser beam printer or a facsimile apparatus utilizing the electrophotographic process, and particularly to an image forming apparatus provided with a contact charging member as a charging means.
2. Related Background Art
In a prior-art image forming apparatus utilizing the electrophotographic process, a corona charger has often been used as means for charging a drum type electrophotographic photosensitive body (hereinafter simply referred to as the photosensitive body) as an image bearing member. The corona charger is disposed in a non-contact and in an opposed relationship with the photosensitive body and the surface of the photosensitive body is exposed to discharge corona created by the corona charger to thereby charge the surface of the photosensitive body to a predetermined polarity and potential.
Also, in recent years, a contact charging device (a direct charging device) has been put into practical use because it has advantages, such as low ozone and low electric power as, compared with the corona charger. The charging member to which a voltage has been applied is brought into contact with the photosensitive body to thereby charge the surface of the photosensitive body to predetermined polarity and potential.
FIG. 8 of the accompanying drawings is a schematic view showing an example of an image forming apparatus using the contact a charging device as charging means. This image forming apparatus is provided with a drum type photosensitive body 101 as an image bearing member as a main constituent, and a charging roller 102 as a contact charging device, an exposure device 103, a developing device 104, a transfer roller 105 and a cleaning blade 106 around the photosensitive body.
In the image forming apparatus thus constructed, during image formation, the photosensitive body 101 is rotatively driven by driving means (not shown) and is charged by the charging roller 102 which is rotated following the photosensitive body 101. Image exposure L by a laser beam is given to the charged photosensitive body 101 by the exposure device 103, whereby an electrostatic latent image conforming to image information inputted is formed, and this electrostatic latent image is developed as a toner image by the developing device 104. The toner image on the photosensitive body 101 is transferred to a transfer material P, such as paper, by the transfer roller 105, and the transfer material P to which the toner image has been transferred is conveyed to a fixating device (not shown), by which the transferred toner image is fixated as a permanently secured image, and the transfer material P is discharged. Also, any untransferred toner adhering to the photosensitive body 101 after the transfer is removed by the cleaning blade 106, and the photosensitive body 101 having had its surface cleaned repetitively enters the next image forming operation.
Also, the charging roller 102 is provided with a pad-like or brush-like cleaning member (not shown), which is adapted to be urged against the charging roller 102 to thereby periodically remove any toner adhering to the charging roller 102 and the scrapings of the photoconductive layer of the photosensitive body 101.
Now, in the above-described prior art image forming apparatus, as the frequency of image formation increases, the surface of the photosensitive body 101 is scraped by the cleaning blade 106 and the developer of the developing device 104, whereby the surface roughness of the photosensitive body 101 gradually becomes greater. Particularly, with regard to the untransferred toner slipping away from the cleaning blade 106, localized slip-away becomes greater in amount as the surface roughness of the photosensitive body 101 becomes greater. As the result, when the frequency of image formation increases and the deterioration of the surface of the photosensitive body 101 progresses, the degree of stain of the charging roller 102 becomes great as compared with the initial stage and thus, cleaning becomes insufficient if only the cleaning member (not shown) of the charging roller 102 is used. Therefore, as the charging roller 102 becomes stained, the irregularity of charging occurs and it becomes impossible to effect good image formation.
As a countermeasure for this, it is conceivable to simply increase the cleaning capability of the cleaning member, but this has led to the problem that the load of the cleaning member to the charging roller 102 becomes great and the surface of the charging roller 102 becomes liable to be damaged and the life of the charging roller 102 becomes short and also the deterioration of the cleaning member is hastened to thereby shorten the life thereof.
It is also conceivable to make the pressure of contact of the cleaning blade 106 with the photosensitive body 101 great in order to prevent the slip-away of the untransferred toner by the cleaning blade 106, but in this case, there has been the problem that the load of the photosensitive body 101 becomes great or the so-called fusing phenomenon that the toner adheres to the surface of the photosensitive body 101 is liable to occur.